Colorimetric and fluorescent on-off detection of Cu2+, Sn2+ and Zn2+ by a water-soluble porphyrin: Electronic absorption and emission study

The strong light absorption behavior and high emission property of porphyrins and their derivatives permit the potential detection of various transition metal ions through binding to the porphyrin core. Here in we report the colorimetric and fluorescent enhancement and fluorescent turn-off detection of Sn2+, Zn2+ and Cu2+ by using a simple water-soluble porphyrin (TPPS) as molecular sensor. The porphyrin sensor displayed different electronic absorption pattern, fluorescent emission behavior and color change up on interacting with Sn2+, Zn2+ and Cu2+ in aqueous solution at room temperature. Interestingly, TPPS showed higher selectivity toward Cu2+ than Sn2+ and Zn2+ ions. Treating aqueous solution of TPPS with a mixture of Sn2+ and Zn2+ ions showed better selectivity for Sn2+ than Zn2+ ion. On the other hand, the electronic absorption pattern and emission behavior of TPPS showed no change at all up on addition of aqueous solution of the rest cations under study (Co2+, Ca2+, Na+, Ag+, Mn2+, Pb2+, Fe3+, Cd2+, Ni2+, NH4+). This may be attributed to unique electronic behavior of the metal ion and ionic radius. The study shows the possibility of molecular based selective metal ion detection in aqueous solution. The selectivity and fluorescence on-off detection may be attributed to the size of metal ions to easily incorporate into porphyrin core, paramagnetic nature of metal ions, conformational rigidity after complexation and inhibition of PET (Photoinduced Electron Transfer) quenching pathways. The study has been successfully applied to effective detection of metal ion from discharged tannery effluent collected from Bahir Dar Tannery before treatment.